Due to chipset limitation, DDR2-800 with CL=4 will be downgraded to run at DDR2-667 by default setting. If you want to operate with lower latency, adjust the memory timing manually.

Some old-version DDR2-800/667 DIMMs, may not match Intel's On-Die-Termination (ODT) requirement and will automatically downgrade to run at DDR2-533. If this happens, contact your memory vendor to check the ODT value.

I can’t find any section on the BIOS menu where I could adjust the memory tiomings manually?!
But this specific Kingston RAM is mentioned as CL5, with timings:5-5-5-15, so this shouldn’t be happening!?

OK, so I disabled SPD and set the right timings: 5,5,5,15 and in the Advanced, "Jumper Free Configuration", I set the DRAM frequency to DDR-800MHZ, so when the system boots up, (at the American Megatrends Bios DOS screen), the memory is indeed recognized as:
PC2-6400 800MHZ and not PC2-5300 667MHz.

But I noticed that when I use CPU-Z 1.38, in the “SPD” field, the two DDR-2 modules are recognised as PC2-5300 (333MHZ)?

Where as in the “Memory” field, the frequency and timings are right: 400MHZ, timings: 5,5,5,15

Sometimes if you buy fast ram that hasnt yet got a Jedec specification, the SPD will show it as the highest previous rated type. So, for example, lets say that pc2-6400 hadnt yet been formally approved as a spec, then even ram that was rated to run at pc2-6400 would still carry pc2-5300 in its SPD.

Im not saying that this is what has happened with yours (date of manufacture rules that out), but if it runs at the speed that it should, then dont worry about it

Sometimes if you buy fast ram that hasnt yet got a Jedec specification, the SPD will show it as the highest previous rated type. So, for example, lets say that pc2-6400 hadnt yet been formally approved as a spec, then even ram that was rated to run at pc2-6400 would still carry pc2-5300 in its SPD.

I think this would be the case.

But what about the ICH8R Southbridge chipset, being recognized as ICH8?

I know this post is a little older, but here is some info that I dug up about this exact issue. I got this data direct from kingston.com:

Some motherboard BIOS may need to be manually adjusted. There are over a million different combinations available. The most common are those that help DRAM speed, DRAM timings, and voltage. CLICK HERE for the proper settings.

Further, when digging even deeper, I found this statemenet for this exact model memory chip (KHX6400D2/1G):

This module has been tested to run at DDR2 800MHz at low latency timing of 5-5-5-15 at 2.0V. The
SPD is programmed to JEDEC standard latency 667Mhz timing of 5-5-5-15 at 1.8V.